On many occasions posts are placed in vortex that discuss the efficiency of
heat engines and cycles as well as whether or not all the heat can be extracted
from a system and so on. A thought occurred to me earlier today which made
much of the confusion go away and I wanted to share that concept with the
others in the group. It is my hope that this unusual way of looking at these
types of problems will simplify these outwardly complex looking systems.
The first thing that needs to be considered is that the conservation of energy
is preserved in these machines and systems. When we speak of efficiency, it is
should not be considered a loss of energy at all, but the lack of ability to
extract all of the energy that is available from the source. The energy that
is not turned into work by the machine is simply returned to the environment
and could be released under the right circumstances.
My thought experiment followed an interesting path. First, think of having an
isolated system such as a resistor in empty space that is at essentially zero
Kelvin and kinetic energy that matches. This resistor has leads attached to
it and we connect a voltage source. According to standard electrical rules,
energy will be given to the resistor at a rate proportional to the power
applied. For example, if 1 watt is being delivered to the resistor, then it is
absorbing energy at a rate of 1 joule per second.
I am quite confident that everyone reading vortex posts understands that the
increase in resistor internal energy is mainly going to be in the form of
thermal energy. This is just a way to characterize kinetic energy of the
molecules of the material. And kinetic energy just means that the atoms are in
motion relative to each other, which can also be measured by the temperature of
the device.
So, after a period of time with power applied to the resistor, it will heat up
and contain a well defined number of joules of energy. I realize that someone
could chase down every last joule of energy in what ever form it takes, but
this is a discussion to help simplify the concept for people wishing a better
understanding of the principles so lets not bring in the secondary processes at
this time.
Someone asked the question as to whether or not the heat within a system could
be mostly extracted and I think we can shed light upon that issue. First of
all, if the external voltage source is removed and the system monitored for a
very long time, it will be seen to radiate heat energy according to the
Stefan-Boltzmann law until it ultimately has none left to radiate. This energy
is in the form of IR radiation initially and eventually changes over to lower
frequency peak emissions until there is no more energy available. Second, the
resistor atoms slowly loose some of their energy of motion (kinetic), so they
move slower. There is no theoretical law that prevents us from capturing most
the radiated energy by one method or the other with a super conductor antenna -
energy conversion device. Of course this would be impractical, but the
principle is there. The end result of this complicated activity would be that
the heat energy has been recovered in some other form and none is lost so the
conservation of energy prevails.
Now, it seems strange that we always speak in terms of two heat sinks when we
talk of the efficiency of a heat engine. If you think carefully about the
processes at work, you will see that this is just a short hand way of saying
that you begin with kinetic energy of the source driving your heat engine,
which is measured by the temperature of the source, and end up by not
extracting all of the kinetic energy. The low temperature sink is the place
where your engine allows the kinetic energy to escape that was not converted
into mechanical work. This is a simple way to think of the engine. It's
design is imperfect since the input kinetic energy of the source does not all
get converted. Thermal radiation can behave as a perfect heat engine, except
that its output is in the form of electromagnetic radiation instead of
mechanical work.
Always remember that energy is energy and that heat energy is just one of many
types available. Generally, there is a process that will convert one form into
another, and some are easier to work with than others. Raw heat is not the
ideal energy form to work with, especially when compared to an easily converted
type such as electrical energy. The heat can be converted into electrical
energy, but the process does not typically function without allowing some of
the input heat energy to escape simple conversion.
And, if you convert electrical energy into mechanical work, such as raising a
heavy load into the air with an electrical motor, some of the input electrical
energy will be converted into that less useful form of heat energy. It is not
lost, but harder to put into use after that process.
